Degradable preservative film for fruit preservation and preparation method thereof

Abstract

The invention discloses a degradable preservative film for fruit preservation and a preparation method thereof, and belongs to the technical field of preservative films. Comprising the following raw materials in parts by weight: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan and 3-9 parts of a lubricant. The prepared preservative film takes polylactic acid as a matrix, so that the preservative film is endowed with relatively high transparency, heat resistance and degradability; wherein the polyvinyl alcohol is a degradable material and can enhance the gas barrier property of the preservative film; wherein the cellulose can improve the thermal stability and the mechanical property of the preservative film; wherein the modified chitosan can enhance the antibacterial property, the heat resistance, the water barrier property and the mechanical property of the preservative film, and the properties are long-lasting and stable; therefore, the prepared preservative film is excellent in mechanical property and good in barrier property, has stable and efficient antibacterial property and heat resistance, is environment-friendly and degradable, and has important application value in the technical field of preservative films.

Description

Technical Field

[0001] The invention belongs to the technical field of cling film, and in particular relates to a degradable cling film for keeping fruits fresh and a preparation method thereof. Background Art

[0002] With the improvement of people's living standards and the acceleration of the pace of life, fresh-cut fruits are becoming more and more popular due to their convenience, health and nutrition. However, since fresh-cut fruits need to be peeled and cut into pieces, the integrity of the fruits has been destroyed and they are very easy to spoil. Therefore, the preservation of fresh-cut fruits has become a hot research direction in recent years. The commonly used method of fruit preservation in life is to put the fruits in the refrigerator for refrigeration or seal them with plastic wrap.

[0003] Cling film is a plastic packaging product, usually made of ethylene as the masterbatch through polymerization reaction. Cling film can be divided into three categories: the first is polyethylene, referred to as PE; the second is polyvinyl chloride, referred to as PVC; the third is polyvinylidene chloride, referred to as PVDC. It is widely used in food packaging fields such as microwave food heating, refrigerator food storage, fresh and cooked food packaging, etc., in family life, supermarkets, hotels and restaurants, and industrial production. However, the commonly used cling film on the market generally does not have antibacterial properties, and there are a large number of microorganisms in nature. These hundreds of millions of tiny organisms are distributed in the air, water, and soil. It can be said that they are everywhere, and it is easy to breed various bacteria on fruits, affecting consumption, especially fruits such as lychees, whose pulp is rich in sugar and water, with high nutritional value, but more prone to rot and deterioration. In addition, in hot seasons such as summer, the cling film needs to have certain heat resistance to ensure the normal use of the cling film; not only that, most cling films are disposable and are usually discarded as garbage after use. Due to the difficulty of recycling and the inability to degrade, they cause serious pollution to the environment. Therefore, it is urgent to solve the above problems and invent a degradable cling film for fruit preservation to meet the higher demands in the field of cling film technology. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a degradable fresh-keeping film for preserving fruits and a preparation method thereof.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A method for preparing a degradable fresh-keeping film for preserving fruits comprises the following steps:

[0007] Polylactic acid, polyvinyl alcohol, cellulose, modified chitosan and lubricant are mixed in a mixer to obtain a mixture; the mixture is then added into a screw extruder to perform hot melt extrusion blow molding to form a film, thereby obtaining a degradable fresh-keeping film for preserving fruits.

[0008] Furthermore, the raw materials are calculated in parts by weight as follows: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan, and 3-9 parts of lubricant.

[0009] Furthermore, the cellulose is one of wood cellulose, fruit cellulose and hemp cellulose.

[0010] Furthermore, the lubricant is one of epoxidized soybean oil and paraffin.

[0011] The prepared cling film is based on polylactic acid, which is a new type of biodegradable material that is not only highly transparent but also has good heat resistance. In addition, polyvinyl alcohol has a large number of hydroxyl groups in the chain, which are easy to form hydrogen bonds. The presence of a large number of hydrogen bonds gives the cling film good mechanical properties and high gas barrier properties, and polyvinyl alcohol is a degradable material. The addition of cellulose can further improve the thermal stability and mechanical properties of the cling film.

[0012] Furthermore, the modified chitosan is prepared by the following steps:

[0013] S1. In a three-necked flask equipped with a stirring reflux device, stearic acid, hydroquinone and toluene are mixed and stirred uniformly, and dicyclohexylcarbodiimide (DCC, dehydrating agent) and 4-dimethylaminopyridine (DMAP, catalyst) are added in sequence, and the reaction temperature is controlled to 80°C, and the reflux reaction is carried out for 8 hours. After the reaction is completed, the mixture is filtered, and part of the solvent is removed by rotary evaporation. The mixture is washed with anhydrous ethanol for 2-3 times, and vacuum dried to obtain intermediate 1; the ratio of the amount of stearic acid, hydroquinone, toluene, dicyclohexylcarbodiimide and 4-dimethylaminopyridine is 28.4g:12.4g:100mL:20.6g:0.2g;

[0014] Under the action of DCC and DMAP, stearic acid and hydroquinone undergo esterification reaction. By controlling the molar ratio of the two to be close to 1:1 and a slight excess of hydroquinone, only one hydroxyl group on the hydroquinone participates in the reaction to obtain intermediate 1. The specific reaction process is as follows:

[0015]

[0016] S2, the intermediate 1 and toluene were mixed, added to a three-necked round-bottom flask with a thermometer, a magnetic stirring system and a spherical condenser, and formaldehyde solution (mass fraction 28%) was added dropwise under ice-water bath conditions, and stirring was continued for 45 minutes, and then diethylenetriamine was added, and the reaction temperature was controlled to 60°C, and the reflux reaction was carried out for 6 hours. After the reaction was completed, it was cooled to room temperature, and the solvent was removed by rotary evaporation. Toluene and deionized water were added to separate the layers, and the upper organic phase was taken, and washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in turn, and the upper organic phase was separated and dried to obtain intermediate 2; the ratio of the amount of intermediate 1, toluene, formaldehyde solution and diethylenetriamine was 37.6g:100mL:20mL:11.7g;

[0017] Intermediate 1 reacts with diethylenetriamine, and intermediate 2 is obtained by controlling the molar ratio of the two to be close to 1:1 and the diethylenetriamine to be slightly excessive. The specific reaction process is as follows:

[0018]

[0019] S3, in a three-necked flask equipped with a stirring device, add intermediate 2 and toluene, stir evenly, then add glutaraldehyde and piperidine (condensing agent), stir and mix evenly, control the reaction temperature to 70 ° C, keep the reaction for 5h, the reaction is completed, and part of the solvent is removed by distillation under reduced pressure, and then purified by column chromatography (eluent adopts a mixed solvent of petroleum ether / ethyl acetate, the volume ratio of the two is 5:1), and the eluent is removed by rotary evaporation to obtain intermediate 3; the ratio of the amount of intermediate 2, toluene, glutaraldehyde, and piperidine is 50.3g:150mL:10.8g:15mL;

[0020] Under the action of the condensing agent, the aldehyde group on glutaraldehyde condenses with the amino group on intermediate 2 to form an imine group (C=N Schiff base structure). By controlling the molar ratio of the two to be close to 1:1 and a slight excess of glutaraldehyde, intermediate 3 is obtained; the specific reaction process is as follows:

[0021]

[0022] S4, in a three-necked flask equipped with a stirring reflux device, the intermediate 3 and toluene were mixed and stirred uniformly, and methyl iodide was added dropwise, and the reaction temperature was controlled to 60°C, and reflux reaction was started. After the addition of methyl iodide was completed, the temperature was raised to 80°C, and the reflux reaction was continued for 8h. After the reaction was completed, the intermediate 4 was obtained by distillation under reduced pressure; the ratio of the amount of intermediate 3, methyl iodide and toluene was 58.5g:42.6g:150mL;

[0023] Intermediate 3 undergoes an alkylation reaction with methyl iodide to obtain intermediate 4 (quaternary ammonium product); the structure of intermediate 4 is shown below:

[0024]

[0025] S5. Add chitosan to a mixed solution of acetic acid and toluene (the mass fraction of acetic acid is 28%), stir until completely dissolved, then add intermediate 4 and piperidine, stir and react at room temperature for 45 minutes, add sodium hydroxide solution dropwise to adjust the pH to 5.5, continue the reaction for 2 hours, and after the reaction is completed, let it stand and precipitate, filter, wash with ethanol and water in turn, and dry in an oven to obtain modified chitosan; the ratio of the mixed solution of acetic acid and toluene, chitosan, intermediate 4, and piperidine is 100mL:1g:10.5g:15mL;

[0026] Under the action of the condensation agent, the aldehyde group on the intermediate 4 condenses with the amino group on the chitosan to form an imine group (C=N Schiff base structure) to obtain modified chitosan;

[0027] Chitosan is a natural biodegradable material with good stability and antibacterial properties. Chitosan is also a high molecular compound with good film-forming properties, which can enhance the mechanical strength of the matrix. By modifying chitosan and connecting it with the organic molecular chain through C=N, the hydrogen bonding effect of the chitosan amino group is weakened, the biological activity and antibacterial properties of chitosan are improved, and the surface hydrophobicity of the modified chitosan is improved, which enhances the compatibility with the matrix and promotes the dispersion of the modified chitosan, so that the performance of the modified chitosan can be fully exerted. In addition, the modified chitosan molecule also contains There are quaternary ammonium salts, long carbon chains, benzene rings and benzoxazine structures. Among them, quaternary ammonium salts are an excellent antibacterial structure with good permeability, low toxicity and stable performance. They penetrate the cell wall to leak intracellular substances, leading to bacterial death, and play a synergistic role with Schiff base and chitosan to significantly enhance the antibacterial properties of the matrix. The introduced long carbon chain is a methylene chain segment, which is a flexible chain segment. It can not only enhance the flexibility of the matrix, but also has hydrophobicity with the benzene ring, which can enhance the waterproofness of the matrix and improve the water barrier performance. Finally, the introduced benzoxazine can enhance the heat resistance of the matrix.

[0028] It should be added that chitosan connects organic molecular chains, which can effectively protect small organic molecules, improve their resistance to migration and exudation, and maintain the long-term stability of the performance of modified chitosan.

[0029] Beneficial effects of the present invention:

[0030] 1. The cling film prepared by the present invention uses polylactic acid as a matrix, which gives the cling film higher transparency, heat resistance and degradability;

[0031] 2. Adding polyvinyl alcohol not only makes it a degradable material, but also enhances the mechanical properties and gas barrier properties of the cling film;

[0032] 3. Adding cellulose can improve the thermal stability and mechanical properties of cling film to a certain extent;

[0033] 4. By modifying chitosan, compared with ordinary chitosan, it has better compatibility with the matrix, can also greatly enhance the antibacterial property, heat resistance, water barrier property and mechanical properties of the cling film to a certain extent, and the performance is long-lasting and stable;

[0034] Therefore, the cling film prepared by the present invention has excellent mechanical properties, high barrier properties to water and gas, stable and efficient antibacterial and heat resistance, and is environmentally friendly and degradable, and has important application value in the field of cling film technology. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] Preparation of modified chitosan:

[0038] S1. In a three-necked flask equipped with a stirring reflux device, 28.4 g of stearic acid, 12.4 g of hydroquinone and 100 mL of toluene were mixed and stirred evenly, and 20.6 g of dicyclohexylcarbodiimide and 0.2 g of 4-dimethylaminopyridine were added in sequence. The reaction temperature was controlled to 80°C, and the reflux reaction was carried out for 8 hours. After the reaction was completed, the reaction was filtered, and part of the solvent was removed by rotary evaporation. The product was washed with anhydrous ethanol for 2-3 times and dried in vacuo to obtain intermediate 1.

[0039] S2, 37.6g of intermediate 1 and 100mL of toluene were mixed, added to a three-necked round-bottom flask equipped with a thermometer, a magnetic stirring system and a spherical condenser, and 20mL of formaldehyde solution (mass fraction 28%) was added dropwise under an ice-water bath, and stirred for 45min, then 11.7g of diethylenetriamine was added, the reaction temperature was controlled to 60°C, and the reaction was refluxed for 6h. After the reaction was completed, the mixture was cooled to room temperature, the solvent was removed by rotary evaporation, toluene and deionized water were added for stratification, the upper organic phase was taken, and it was washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in sequence, the upper organic phase was separated, and dried to obtain intermediate 2;

[0040] S3, in a three-necked flask equipped with a stirring device, add 50.3g of intermediate 2 and 150mL of toluene, stir evenly, then add 10.8g of glutaraldehyde and 15mL of piperidine, stir and mix evenly, control the reaction temperature to 70°C, keep the reaction for 5h, the reaction is completed, and part of the solvent is distilled off under reduced pressure, and then purified by column chromatography (eluent adopts a mixed solvent of petroleum ether / ethyl acetate, the volume ratio of the two is 5:1), and the eluent is removed by rotary evaporation to obtain intermediate 3;

[0041] S4, in a three-necked flask equipped with a stirring reflux device, 58.5g of intermediate 3 and 150mL of toluene were mixed and stirred evenly, and then 42.6g of methyl iodide was added dropwise, and the reaction temperature was controlled to 60°C, and reflux reaction was started. After the addition of methyl iodide was completed, the temperature was raised to 80°C, and the reflux reaction was continued for 8h. After the reaction was completed, the intermediate 4 was obtained by distillation under reduced pressure;

[0042] S5. Add 1 g of chitosan to a mixed solution of 100 mL of acetic acid and toluene (the mass fraction of acetic acid is 28%), stir until completely dissolved, then add 10.5 g of intermediate 4 and 15 mL of piperidine, stir and react at room temperature for 45 min, add dropwise sodium hydroxide solution to adjust the pH to 5.5, continue the reaction for 2 h, and after the reaction is completed, let stand and precipitate, filter, wash with ethanol and water in turn, and dry in an oven to obtain modified chitosan.

[0043] Example 2

[0044] Preparation of modified chitosan:

[0045] S1. In a three-necked flask equipped with a stirring reflux device, 56.8 g of stearic acid, 24.8 g of hydroquinone and 200 mL of toluene were mixed and stirred evenly, and 41.2 g of dicyclohexylcarbodiimide and 0.4 g of 4-dimethylaminopyridine were added in sequence. The reaction temperature was controlled to 80°C, and the reflux reaction was carried out for 8 hours. After the reaction was completed, the reaction was filtered, and part of the solvent was removed by rotary evaporation. The product was washed with anhydrous ethanol twice and dried in vacuo to obtain intermediate 1.

[0046] S2, 75.2g of intermediate 1 and 200mL of toluene were mixed, added to a three-necked round-bottom flask equipped with a thermometer, a magnetic stirring system and a spherical condenser, 40mL of formaldehyde solution (mass fraction 28%) was added dropwise under ice-water bath conditions, and stirring was continued for 45min, then 23.4g of diethylenetriamine was added, the reaction temperature was controlled to 60°C, refluxed for 6h, the reaction was completed, cooled to room temperature, the solvent was removed by rotary evaporation, toluene and deionized water were added for stratification, the upper organic phase was taken, and it was washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in sequence, the upper organic phase was separated, and dried to obtain intermediate 2;

[0047] S3, in a three-necked flask equipped with a stirring device, add 100.6g of intermediate 2 and 300mL of toluene, stir evenly, then add 21.6g of glutaraldehyde and 30mL of piperidine, stir and mix evenly, control the reaction temperature to 70°C, keep the reaction for 5h, the reaction is completed, and part of the solvent is distilled off under reduced pressure, and then purified by column chromatography (eluent adopts a mixed solvent of petroleum ether / ethyl acetate, the volume ratio of the two is 5:1), and the eluent is removed by rotary evaporation to obtain intermediate 3;

[0048] S4, in a three-necked flask equipped with a stirring reflux device, 117g of intermediate 3 and 300mL of toluene were mixed and stirred uniformly, and then 85.2g of methyl iodide was added dropwise, and the reaction temperature was controlled to 60°C, and reflux reaction was started. After the addition of methyl iodide was completed, the temperature was raised to 80°C, and the reflux reaction was continued for 8h. After the reaction was completed, the intermediate 4 was obtained by distillation under reduced pressure;

[0049] S5. Add 2 g of chitosan to a mixed solution of 200 mL of acetic acid and toluene (the mass fraction of acetic acid is 28%), stir until completely dissolved, then add 21 g of intermediate 4 and 30 mL of piperidine, stir and react at room temperature for 45 min, add sodium hydroxide solution dropwise to adjust the pH to 5.5, continue the reaction for 2 h, and after the reaction is completed, let it stand and precipitate, filter, wash with ethanol and water in turn, and dry in an oven to obtain modified chitosan.

[0050] Example 3

[0051] 40g of polylactic acid, 15g of polyvinyl alcohol, 6g of lignocellulose, 10g of the modified chitosan prepared in Example 1 and 3g of paraffin are mixed in a mixer to obtain a mixture; the mixture is then added to a screw extruder and subjected to hot-melt extrusion blow molding at 160°C to obtain a degradable fresh-keeping film for preserving fruits.

[0052] Example 4

[0053] 50 g of polylactic acid, 20 g of polyvinyl alcohol, 9 g of fruit cellulose, 15 g of the modified chitosan prepared in Example 2 and 6 g of epoxidized soybean oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder and subjected to hot-melt extrusion blow molding at 170° C. to obtain a degradable fresh-keeping film for preserving fruits.

[0054] Example 5

[0055] 60g of polylactic acid, 25g of polyvinyl alcohol, 12g of fruit cellulose, 20g of the modified chitosan prepared in Example 2 and 9g of epoxidized soybean oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder and subjected to hot-melt extrusion blow molding at 180°C to obtain a degradable fresh-keeping film for preserving fruits.

[0056] Comparative Example 1

[0057] Ordinary chitosan of the same mass was used to replace the modified chitosan in Example 5, and the remaining steps were the same as in Example 5 to prepare a fresh-keeping film.

[0058] Comparative Example 2

[0059] Commercially available polylactic acid biodegradable food wrap was used.

[0060] Examples 3-5 and comparative examples 1-2 were made into corresponding shapes according to different test standards, and the following performance tests were performed:

[0061] The tensile strength is measured according to the national standard GB / T 1040-2006 "Determination of tensile properties of plastics"; the sample is then placed at 150°C for 12 hours for heat treatment, the tensile strength is measured, and the tensile strength retention rate is calculated; tensile strength retention rate = tensile strength after test / tensile strength before test × 100%;

[0062] The biodegradation rate was determined using the national standard GB / T 19811-2005;

[0063] QB / T 2591-2003 "Antibacterial plastics - Antibacterial performance test methods and antibacterial effects" was used to measure the antibacterial rate of the sample before and after being placed at 20℃±5℃ and humidity of 10%±5%RH for 30 days;

[0064] The oxygen permeation rate is determined using the national standard GB / T 1038.1-2022 "Test method for gas permeability of plastic film and sheeting Part 1: Differential pressure method";

[0065] The water vapor transmission rate was determined using the national standard GB / T 1037-2010 "Test method for water vapor permeability of plastic film and sheet - cup method";

[0066] The measurement results are shown in Table 1:

[0067] Table 1

[0068]

[0069]

[0070] 500 lychees of uniform size, relatively consistent maturity (8-9 mature), free of rot, pests and inorganic damage were selected, and the lychees in each group were evenly divided into 5 groups, each with 100 lychees. The lychees in each group were packaged with the cling film of Examples 3-5 and Comparative Examples 1-2 respectively; the 5 groups of fruits were then stored in a climate box at 25° C. The good fruit rate of each group of lychees was detected on the 1st day, the 3rd day, the 5th day, the 8th day and the 10th day;

[0071] The measured results are shown in Table 2:

[0072] Table 2

[0073]

[0074] It can be seen from the above two tables that the cling film prepared in the embodiment of the present invention has higher mechanical properties, heat resistance, antibacterial properties and barrier properties than the control example, and the antibacterial property is long-lasting and stable, environmentally friendly and degradable, and has a good preservation effect on litchi, and has important application value in the field of cling film technology.

[0075] In the description of the specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0076] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims

1. A method for preparing a degradable preservative film for preserving fruits, characterized in that: The following steps are involved: Polylactic acid, polyvinyl alcohol, cellulose, modified chitosan and lubricant are mixed in a mixer to obtain a mixture; the mixture is then added into a screw extruder, and then hot-melt extruded and blown into a film to obtain a degradable fresh-keeping film for preserving fruits.

2. The method for preparing a degradable preservative film for preserving fruits according to claim 1, characterized in that: The modified chitosan is prepared by the following steps: S1, mixing stearic acid, hydroquinone and toluene, adding dicyclohexylcarbodiimide and 4-dimethylaminopyridine in sequence, reflux at 80°C for 8h, and after the reaction is completed, filtering, rotary evaporation, washing and vacuum drying are performed to obtain intermediate 1; S2, mixing intermediate 1 and toluene, adding to a round-bottom flask, adding formaldehyde solution dropwise under ice-water bath conditions, and stirring continuously for 45 minutes, then adding diethylenetriamine, reflux reaction at 60°C for 6 hours, and after the reaction is completed, cooling, rotary evaporation, adding toluene and deionized water for layering, taking the upper organic phase, washing, separating the upper organic phase, and drying to obtain intermediate 2; S3, after the intermediate 2 and toluene are stirred evenly, glutaraldehyde and piperidine are added, the mixture is stirred evenly, and the mixture is reacted at 70° C. for 5 h. After the reaction is completed, the mixture is distilled under reduced pressure, purified by column chromatography, and rotary evaporated to obtain the intermediate 3; S4, after mixing the intermediate 3 and toluene, add iodomethane dropwise, and reflux at 60°C for reaction until the addition of iodomethane is complete, then raise the temperature to 80°C, and continue the reflux reaction for 8h. After the reaction is complete, perform distillation under reduced pressure to obtain the intermediate 4; S5. Add chitosan to the mixed solution of acetic acid and toluene, stir until completely dissolved, then add intermediate 4 and piperidine, stir and react at room temperature for 45 minutes, adjust the pH to 5.5, continue the reaction for 2 hours, and after the reaction is completed, let it stand and precipitate, filter, wash, and dry to obtain modified chitosan.

3. The method for preparing a degradable preservative film for preserving fruits according to claim 2, characterized in that: In step S1, the ratio of the amount of stearic acid, hydroquinone, toluene, dicyclohexylcarbodiimide, and 4-dimethylaminopyridine is 28.4 g: 12.4 g: 100 mL: 20.6 g: 0.2 g.

4. The method for preparing a degradable preservative film for preserving fruits according to claim 2, characterized in that: In step S2, the ratio of the amount of intermediate 1, toluene, formaldehyde solution and diethylenetriamine is 37.6 g: 100 mL: 20 mL: 11.7 g.

5. The method for preparing a degradable preservative film for preserving fruits according to claim 2, characterized in that: In step S3, the ratio of the amount of intermediate 2, toluene, glutaraldehyde and piperidine is 50.3 g:150 mL:10.8 g:15 mL.

6. The method for preparing a degradable preservative film for preserving fruits according to claim 2, characterized in that: In step S4, the ratio of the amount of intermediate 3, methyl iodide and toluene is 58.5 g:42.6 g:150 mL.

7. The method for preparing a degradable preservative film for preserving fruits according to claim 2, characterized in that: In step S5, the ratio of the mixed solution of acetic acid and toluene, chitosan, intermediate 4, and piperidine is 100 mL: 1 g: 10.5 g: 15 mL.

8. The method for preparing a degradable preservative film for preserving fruits according to claim 1, characterized in that: The raw materials are calculated in parts by weight as follows: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan, and 3-9 parts of lubricant.

9. The method for preparing a degradable preservative film for preserving fruits according to claim 1, characterized in that: The cellulose is one of wood cellulose, fruit cellulose and hemp cellulose.

10. A degradable fresh-keeping film for preserving fruits, characterized in that: Prepared according to the method according to any one of claims 1 to 9.

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